A storage server is a special-purpose processing system used to store and retrieve data on behalf of one or more client processing systems (“clients”). A storage server can be used for many different purposes, such as, to provide multiple users with access to shared data or to backup mission critical data.
A file server is an example of a storage server. A file server operates on behalf of one or more clients to store and manage shared files in a set of mass storage devices, such as magnetic or optical storage based disks or tapes. The mass storage devices may be organized into one or more volumes of Redundant Array of Inexpensive Disks (RAID). Another example of a storage server is a device which provides clients with block-level access to stored data, rather than file-level access, or a device which provides clients with both file-level access and block-level access.
In conventional file servers, there is a fixed relationship between a volume and the disks that are associated with the volume. This fixed relationship means that each volume has full control over the disks that are associated with the volume. For example, only the volume associated with the disk can read and/or write to the disk. Unused space within the disks associated with the volume cannot be used by another volume. Thus, even if a volume is only using a fraction of the space on its associated disks, the unused space is reserved for the exclusive use of the volume.
Another feature of the fixed relationship between a volume and its associated disks, in conventional file servers, is that the size of the volume is equal to the capacity of the associated disks. This means that the size of the underlying physical storage dictates the size of the volume. If a user wants to increase the size of the conventional volume, she must add disks to the physical storage (e.g., add a RAID group). Similarly, if the user wants to decrease the size of the conventional volume, she must remove disks from the physical storage.